Cerium activated colquirite laser crystal

ABSTRACT

A laser crystal for a solid state laser with tunable emission in the ultraviolet wavelength region. The crystal is of trivalent cerium cations activated (doped) into the colquiriite crystal structures, such as LiCaAlF 6  (LiCAF) and LiSrAlF 6  (LiSAF). The crystal is grown along a well-defined axis by seeded crystal growth techniques to provide starting material which is fabricated along the preferred polarization direction with high yields. A primary crystal growth issue for cerium activated colquiriites is the charge compensation mandated by the substitution of the trivalent cerium ion for divalent strontium, which is located in the only site large enough to support the cerium cation. Charge compensation with monovalent or divalent cations is utilized to optically stabilize the colquiriite crystal and produce a solid state laser host which is less susceptible to color center or optically-induced defect formation during the ultraviolet lasing or ultraviolet excitation mechanisms.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.F19629-94-C-0134 awarded by the Department of Defense/Air Force MaterialCommand.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuing application of Provisional Application Number60/005,958 filed on Oct. 27, 1995 (pending).

BACKGROUND OF INVENTION

The invention relates most generally to the crystal growth and chargecompensation techniques of light-transmitting solid-state opticalmaterials, more particularly to the crystal growth and chargecompensation techniques of rare-earth-ion-doped solid-state laser hostcrystals, and most particularly to the crystal growth and chargecompensation techniques of cerium-doped (activated) colquiriitecrystals.

An interest in tunable solid state laser systems over the last fewdecades has resulted in the discovery of many laser host crystals dopedwith transition metal ions, including Ti:Al₂ O₃, Co:MgF₂, Cr:BeAl₂ O₄,and Cr:LiSrAlF₆ and its isomorphs. These crystals exhibit broad bandlaser emission from the red into the near-infrared portions of thespectrum due to vibronic transitions within the 3d shell. Typicaltransitions within the 4f shell of the trivalent rare earth ions, suchas Nd³⁺, Ho³⁺, and Er³⁺, lead to narrow-band emission in the same red tonear-infrared range. The typical host crystals for these rare earth ionsare Y₃ Al₅ O₁₂ and LiYF₄. In an effort to encompass the completespectrum, emission in the shorter wavelength region has been sought withthe 5d-4f transitions in the divalent rare earths, as well as intrivalent cerium.

The electronic configuration of the Ce³⁺ ion consists of a palladiumcore of 46 electrons with outer electrons in the followingconfiguration: 4f¹ 5s² 5p⁶ 5d¹ 6s². In its trivalent ionic state, theunfilled inner 4f shell is shielded by the 5s and 5p shells. There aretwo ground state levels in the 4f shell, which result from a spin-orbitinteraction splitting. The ² F_(7/2) upper level is locatedapproximately 2000 cm⁻¹ above the ² F_(5/2) ground state. A singleelectron is located in the 5d shell, 5d levels oughly 50,000 cm⁻¹ abovethe ground level (35,000 cm⁻¹ in oxide hosts), which is very sensitiveto different crystal fields. The nature of the 5d wavefunction resultsin a vibrationally broadened emission spectrum, as compared to thenarrow electric dipole transitions within the 4f shell. Unfortunately,if the 5d level is close to the excited 4f levels, the 5d level willdepopulate and result in no broadband emission. Even if emission occurs,excited state absorption, color center formation, and solarization, alldetrimental to the production of a viable solid state laser, havepreviously been observed in many Ce³⁺ -doped laser hosts, includingLiYF₄ Ehrlich, et al., "Ultraviolet solid-state Ce:YLF laser at 325 nm,"Optics Letters 4 (6), 184-186 (1979)!, Y₃ Al₅ O₁₂ Jacobs, et al.,"Measurements of excited-state-absorption loss for Ce³⁺ in Y₃ Al₅ O₁₂and implications for tunable 5d-4f rare-earth lasers," Applied PhysicsLetters 33 (5), 410-412 (1978)!. and CaF₂ Pogatshnik, et al.,"Excited-state photoionization of Ce³⁺ ions in Ce³⁺ :CaF₂," PhysicalReview B 36 (16), 8251-8257 (1987)!.

Tunable laser emission in the ultraviolet region is currently beingachieved commercially with dye lasers and optical parametricoscillators. Unfortunately, their size, complexity, and toxicity (dyes)make them undesirable for many applications. The development of solidstate lasers in this ultraviolet region would have a wide range ofapplications, including atmospheric remote sensing of chemical species,such as ozone.

The colquiriite family of laser host crystals includes LiCaAlF₆,LiSr_(1-x) Ca_(x) F₆, LiSrAlF₆, LiSrGaF₆, and other isomorphs Viebahn,"Untersuchungen an quaternaren Fluoriden LiMe^(II) Me^(III) F₆, DieStruktur von LiCaAlF₆," Zeitschrift fur anorganische und allgemeineChemie. Band 386, 335-339 (1971)!. Cr³⁺ -doped LiCaAlF₆ was firstinvestigated and developed at Lawrence Livermore National Laboratory(LLNL) in 1988 Payne, et al., "LiCaAlF₆ :Cr³⁺ : A promising newsolid-state laser material," IEEE Journal of Quantum Electronics 24(11), 2243-2252 (1988)!. Cr:LiCaAlF₆ is a tunable laser material with atuning range from 720 to 840 nm, similar to that of alexandrite, Cr³⁺:BeAl₂ O₄. Cr³⁺ :LiSrAlF₆, an isomorph achieved by the full substitutionof strontium for calcium, exhibits absorption peaks at approximately 450and 650 nm and a broadband stimulated emission curve spanning from 780to 1010 nm. Both laser crystals can be efficiently pumped by a varietyof techniques, due to the characteristically broad chromium absorptionbands in the visible region. The longer excited state lifetimes, 67 μsfor Cr:LiSrAlF₆, make these materials especially suitable for flashlamppumping and energy storage when compared to Ti³⁺ :Al₂ O₃ with its 3 μslifetime. These chromium-doped colquiriite hosts also have low thermallensing and do not exhibit concentration quenching. Since Cr:LiSrAlF₆spans the same wavelength region as Ti:Al₂ O₃ and has many favorableproperties, it has become an undeniable competitor for Ti:Al₂ O₃.

The success or failure of a potential laser crystal doped with any iondepends on the crystal field, the size and charge of the site designatedfor substitution, as well as many subtle details regarding theinteraction of the impurity ion with the crystal lattice. Recently therehas been a great deal of interest in the doping of fluoride crystals ofthe colquiriite type with the cerium ion for the production of a laseremitting in the ultraviolet region.

In 1993, the first report was made of an ultraviolet-generating gainmedia which lased reliably, that being Ce-doped LiCaAlF₆ M. A.Dubinskii, et al. "Ce³⁺ -doped colquiriite. A new concept ofall-solid-state tunable ultraviolet laser," Journal of Modern Optics 40,(1) 1-5 (1993); M. A. Dubinskii, et al., "Spectroscopy of a New ActiveMedium of a Solid State UV Laser with Broadband Single-Pass Gain," LaserPhysics 3, (1) 216-217 (1993)!. In these published results, anon-oriented section of Ce:LiCaAlF₆ was pumped with the fourth harmonic266 nm output of a Nd:YAG laser and was demonstrated to yield gain andto provide laser oscillation when appropriately arranged in a cavity.However, neither the effect of charge compensating cations nor thepolarization of the pump source were considered (with no preferredpolarization even indicated).

Several patents are relevant to the art. The host employed by Dubinskii,et al., LiCaAlF₆, was disclosed in U.S. Pat. No. 4,811,349, althoughwith Cr³⁺ dopants. U.S. Pat. No. 4,233,570 discusses the potential useof flashlamps to pump Ce³⁺ -laser materials having certaincharacteristics. U.S. Pat. No. 4,132,962 discloses lasers with Ce-dopedfluorides that contain a metal ion from column IIIB of the periodictable. Likewise, U.S. Pat. No. 4,083,018 describes Ce:LiYF₄, Ce:LaF₃,and other related crystals. Finally, two recent device patents haveissued which described unique resonator designs which utilizeCe-activated colquiriite hosts. The first of these, U.S. Pat. No.5,487,079, discloses a Ce:LiSAF laser device designed for atransverse-optically pumped geometry. The second recent patent, U.S.Pat. No. 5,517,516, discloses Ce-doped LiSAF and LiCAF devices which canachieve extremely high laser efficiencies through end-pumpinggeometries, with appropriately selected polarization coupling foroptimum performance. However, neither of these most recent patentsdiscusses crystal growth techniques, charge compensation, or defectminimization schemes. It is also worth noting that both of these patentsare based upon using the Ce-activated colquiriite laser gain materialsthat were grown and developed by the inventors listed in this patent.

Many laser device engineers work diligently to optically pump a lasersystem as efficiently as possible in anticipation of maximizing theoutput. However, as is demonstrated in the results for this patent, onemust also consider the optimization of the crystal growth process andminimization of the defects introduced during the optimized crystalgrowth technique. By reducing these defects, the laser engineers canthen design their laser systems with fewer variable to manipulate inorder to achieve the highest efficiency possible.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a singlecrystal optical matter capable of transmitting light.

It is another object of the invention to provide a single-crystal hostmaterial that may be grown via a seeded growth technique, and can befabricated along an orientation direction which is preferable for laseroperation.

It is another object of the invention to provide a single-crystal hostmaterial that may be doped with metal cations.

It is also an object of the invention to provide a solid-state lasergain medium, comprising a fluoride-based colquiriite crystal that isdoped with metal cations.

It is a further object of the invention to provide a cerium and metalcation co-doped solid-state colquiriite laser crystal that exhibitssuperior tunable ultraviolet laser properties with minimized defects.

These and additional objects of the invention are accompanied by thestructures and processes hereinafter described.

The invention comprises cerium-doped colquiriite crystals, typically tobe grown through a seeded-growth technique. This class of crystal hostscan be co-doped with cerium and another metal cation for improved laserperformance. The co-doping by the cations facilitates more uniformdoping of the cerium activator cation into the lattice and minimizesdefects which impact the tunable ultraviolet laser performance. Thesedefects include intrinsic bulk defects (such as scatter andparticulates) and light activated or induced defects (such as colorcenters).

BRIEF DESCRIPTION OF THE DRAWINGS

No drawings have been attached as part of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention described herein is a solid state laser materialcomprising trivalent cerium cation doping of the colquiriite structurehosts, such as LiCaAlF₆, LiSr_(1-x) Ca_(x) F₆, LiSrAlF₆, LiSrGaF₆, andother isomorphs. In order to produce the highest efficiency ultravioletlaser output, two key components of this invention must be instituted.First, this new class of materials must be grown along a well-definedaxis by seeded crystal growth techniques to provide starting materialwhich can be fabricated along the preferred polarization direction withhigh yields. Growth of the colquiriite boules off-axis can causesignificant problems with decreasing yields through less than efficientextraction of the laser samples. Likewise, unseeded growth can lead tosections of the boule which are randomly oriented, thus impacting (andusually diminishing) the total laser efficiency of gain media extractedfrom these regions Marshall, et al., "Ultraviolet laser emissionproperties of Ce³⁺ -doped LiSrAlF₆ and LiCaAlF₆," Journal of the OpticalSociety of America B 11 (10), 2054-2065, (1994)!. Another primarycrystal growth issue for cerium activated colquiriites is the chargecompensation mandated by the substitution of the trivalent cerium ionfor divalent strontium, which is located in the only site large enoughto support the cerium cation. Charge compensation with monovalent ordivalent cations is utilized and described herein to optically stabilizethe colquiriite crystal and produce a solid state laser host which isless susceptible to color center or optically-induced defect formationduring the ultraviolet lasing or ultraviolet excitation mechanisms.

The colquiriite crystals detailed herein are synthetically grown aslarge single crystals by utilizing known techniques such as theCzochralski method. Generally, rare earth dopant concentrations ofapproximately 1 atomic percent in the melt are used for the growth ofthe cerium-doped crystals, and more generally about 0.01 to 10 atomicpercent in the melt. Crystals having a nominal diameter of 2 mm andlength of approximately 18 cm have been formed, and can probably be madelarger. The colquiriite crystals are made by first mixing anear-stoichiometric chemical charge with nearly equal proportions of,for example, commercially available high purity powders of LiF, SrF₂,AlF₃, with a percentage of SrF₂ substituted with CeF₃ and a percentageof SrF₂ or AlF₃ substituted with the charge compensating material (forexample, the chemical for LiSAF are mixed with a 1.01:1.00:1.02 Li:SR:Alatomic % ratio). These starting materials are then melted and held atthe melting point, ranging from 750° C. to 850° C. for the variouscolquiriite isomorphs. A seed is dipped into the melt, as prescribed bythe Czochralski technique, and then rotated at the rate between 16 and19 rpm, depending on crystal diameter, while being slowly raised at therate of less than 1 mm/hr, generally about 0.6 mm/hr. A crystallineboule thereby slowly emerges from the melt, and most of the boule isfound to consist of clear single crystal material, with minimal cracks,smoke, or other forms of optical defects. Crystals grown utilizing thesegrowth parameters typically exhibited passive losses of less than0.1%/cm, which is adequate for most laser applications.

To understand the requirements for proper crystal growth and chargecompensation one must understand the crystal lattice structure and ioncharge balance requirements. The crystal lattice of the colquiriitesbelongs to the space group D² _(3d) with two formula units per unit celland is derived from the Li₂ ZrF₆ structure Schaffers, et al., "Structureof LiSrAlF₆," Acta Crystallographica C47, 18-20 (1991)!. The trigonalcolquiriite structure is such that each metal ion site is unique in sizeand charge. Surrounded by fluorine ions, each of the three metal ionsare located in distorted octahedral sites, with a D₃ site symmetrybetween planes of close-packed fluorine anions. These metal cations alldiffer both in size and in charge. The cerium activator ion carries a 3+charge, is 1.15 Å in ionic radius, and has a 4f¹ electron configuration.The size of the Ce³⁺ ionic radius would lead one skilled in the art topresume that rare earth ion dopants, such as Ce³⁺, would preferentiallysubstitute into the M^(II) divalent (Sr²⁺ /Ca²⁺) sites (1.27 Å/1.14 Å),rather than the 0.67 Å Al³⁺ site. The lithium site has a prohibitivelylarge mismatch in both charge (a 1+ charge) and in ionic radius and size(0.73 Å). Even though the divalent sites are sufficiently large toaccommodate the trivalent cerium ion, there is a charge imbalance whichdemands that compensation be present somewhere else in the lattice tomaintain charge neutrality.

Charge compensation can be achieved by physically adding an impurity ionwith a different charge designated to substitute into a particular siteby its size. There are two particular types of compensation techniquesthat are addressed in the claims of this patent. The first techniqueinvolves the use of monovalent cations (such as Na¹⁺) on a divalent site(such as Sr²⁺) so that the extra 1+ charge existing from the Ce³⁺ ion onthe divalent site is compensated for by the reduced 1+ charge on anearby divalent site. The second technique involves the use of divalentcations (such as Mg²⁺) on a trivalent site (such as Al³⁺) so that theextra 1+ charge existing from the Ce³⁺ ion on the divalent site iscompensated for by the reduced 1+ charge on a nearby trivalent site. Ifthis charge imbalance is not corrected, anions (such as F⁺) can becometrapped as defects of interstitial anions and produce coloring ordarkening of the crystals when illuminated with ultraviolet photons.This will ultimately raise the threshold of these crystals during laseroperation, reduce the laser output efficiency, and decrease theavailable laser tuning region.

Charge compensation with Na¹⁺, or other monovalent cations, bysubstitution into the Sr²⁺ -type divalent site (1.27 Å) is perhaps themost logical charge compensation step in order to achieve a higher ormore uniform cerium concentration in the crystal, and to minimize thecharge imbalance defects. An ionic radius of 1.16 Å should allow sodiumto easily substitute into the strontium site to compensate for theexcess positive charge in the lattice, resulting from the Ce³⁺incorporation into the Sr²⁺ site. The other monovalent chargecompensation cations are somewhat larger in ionic radii (1.52, 1.63, and1.84 Å for K¹⁺, Rb¹⁺ and Cs¹⁺, respectively), thus reducing theiroverall effect as compensators as their radius mismatch increases. Thecerium distribution should be nearly uniform from the top to the bottomof boules compensated with the Na¹⁺ cation.

The selection process for an appropriate divalent compensator iscomplicated by the fact that this divalent ion must be small enough tofit into the 0.67 Å aluminum site to avoid an undesired substitutiondirectly into the strontium-type site (1.27 Å). Magnesium is a potentialcharge compensating divalent ion for substitution into the Al³⁺ site.Mg²⁺ is electronegative and ionic, however, with an ionic radius of 0.86Å, which would not make this ion 100% effective, primarily because itwould have to enter the aluminum site with nearly a 30% size mismatch.Another divalent cation, Zn²⁺, could also be utilized as thecompensating ion, intended for substitution into the Al³⁺ site. Zinc ismore covalent than magnesium and less electronegative, making it moresimilar to aluminum. Although the nature of the site may be moresuitable for Zn²⁺, the 0.89 Å ionic radius of zinc may cause theintended charge balancing to also be less than 100% effective. Otherdivalent cations that could potentially assist in offsetting any chargeimbalance include Ba²⁺, Be²⁺, and Cd²⁺.

Changes and modifications in the specifically described embodiments canbe carried out without departure from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

What is claimed is:
 1. A laser crystal of the composition M^(I) M^(II)M^(III) F₆, wherein each of said M^(I) sites contains a monovalentcation selected from the group consisting of: Li¹⁺, Na¹⁺, K¹⁺, Rb¹⁺, andC_(S) ¹⁺, C¹⁺, at least one of said M^(II) sites contains a divalentcation selected from the group consistent of: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺,Cd²⁺, and Ba²⁺, at least one said M^(III) sites containing a trivalentcation selected from the group consisting of: B³⁺, Al³⁺, Ga³⁺, Sc³⁺, andIn³⁺, and wherein said laser crystal is doped with trivalent ceriumcations and charge compensating ions selected from the group consistingof: Na¹⁺, K¹⁺, Mg²⁺, Ba²⁺, Be²⁺, Cd²⁺, and Zn²⁺ so that at least one ofsaid M^(II) sites contains Ce³⁺, and one of said M^(II) and M^(III)sites adjacent to each M^(II) site which contains a Ce³⁺ cation containsone of said charge compensating ions.
 2. A laser crystal of thecomposition M^(I) M^(II) M^(III) F₆, wherein each of said M^(I) sitescontains a monovalent cation selected from the group consisting of:Li¹⁺, Na¹⁺, K¹⁺, Rb¹⁺, and C¹⁺, at least one of said M^(II) sitescontains a divalent cation selected from the group consisting of: Mg²⁺,Ca²⁺, Sr²⁺, Cd²⁺, and Ba²⁺ and at least one of said M^(III) sitescontains a trivalent cation selected from the group consisting of: B³⁺,Al³⁺, Ga³⁺, Sc³⁺, and In³⁺, and wherein said laser crystal is doped withtrivalent cerium cations and charge compensating ions selected from thegroup consisting of: Na¹⁺, K¹⁺, so that at least one of said M^(II)sites contains Ce³⁺, and one of said M^(II) sites adjacent each M^(II)site which contains a Ce³⁺ cation contains one of said chargecompensating ions.
 3. A laser crystal of the composition M^(I) M^(II)M^(III) F₆, wherein each of said M^(I) sites contains a monovalentcation selected from the group consisting of: Li¹⁺, Na¹⁺, K¹⁺, Rb¹⁺, andC¹⁺, at least one of said M^(II) sites contains a divalent cationselected from the group consisting of: Mg²⁺, Ca²⁺, Sr²⁺, Cd²⁺, and Ba²⁺and at least one of said M^(III) sites contains a trivalent cationselected from the group consisting of: B³⁺, Al³⁺, Ga³⁺, Sc³⁺, and In³⁺,and, wherein said laser crystal is doped with trivalent cerium cationsand charge compensating ions selected from the group consisting of:Mg²⁺, Be²⁺, Cd²⁺, and Zn²⁺ so that at least one of said M^(II) sitescontains Ce³⁺, and one of said M^(III) sites adjacent each M^(II) sitewhich contains a Ce³⁺ cation contains one of said charge compensatingions.
 4. A laser crystal of the composition M^(I) M^(II) M^(III) F₆,wherein each of said M^(I) sites contains a monovalent cation selectedfrom the group consisting of: Li¹⁺, Na¹⁺, K¹⁺, Rb¹⁺, and C¹⁺, at leastone of said M^(II) sites contains a divalent cation selected from thegroup consisting of: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cd²⁺, and Ba²⁺ and at leastone of said M^(III) sites contains a trivalent cation selected from thegroup consisting of: B³⁺, Al³⁺, Ga³⁺, Sc³⁺, and In³⁺, and where at leastone of said M^(II) sites contains a trivalent cerium cation, and one ofsaid M^(II) and M^(III) sites adjacent each M^(II) site which contain aCe³⁺ cation contains a charge compensating ion selected from the groupconsisting of: Na¹⁺, K¹⁺, Mg²⁺, Be²⁺, Cd²⁺, and Zn²⁺.
 5. A laser crystalas recited in claim 4, wherein said crystal has a trigonal colquiriitelattice structure comprised of formula unit cells having a space groupD² _(3d) with two formula units per unit cell.
 6. A laser crystal asrecited in claim 5, wherein each of said M^(I), M^(II), and M^(III) ionsis surrounded by fluorine anions in distorted octahedral sites.
 7. Alaser crystal as recited in claim 4, wherein said monovalent cation isLi¹⁺, said divalent cation is Sr²⁺, said trivalent cation is Al³⁺ withan atomic percentage of Sr²⁺ substituted with Ce³⁺ and an atomicpercentage of at least one of said Sr²⁺ and Al³⁺ being substituted withat least one of said charge compensating ions, the atomic percentage ofsaid charge compensating ions being equal to the atomic percentage ofsaid Ce³⁺.
 8. A laser crystal as recited in claim 7, wherein the atomicpercentage of each of said Ce³⁺ and said charge compensating ions isfrom 0.01 to 10 atomic percent.
 9. A method of growing a laser crystalof the composition M^(I) M^(II) M^(III) F₆ comprising the followingsteps:(a) providing a quantity of a first compound selected from thegroup LiF, NaF, KF, RbF, and CsF; (b) providing a quantity of a secondcompound selected from the group: BeF², MgF₂, CaF₂, SrF₂, CdF₂, and BaF₂; (c) providing a quantity of a third compound selected from the group:BF³, AlF₃, GaF₃, ScF₃, and InF₃ ; (d) providing a quantity of a fourthcompound of CeF₃ ; (e) providing a quantity of a fifth chargecompensating compound selected from the group: NaF and KF; (f) forming aprimary mixture by mixing substantially equal atomic unit amounts ofsaid first compound, said third compound and a sub mixture of saidsecond, fourth and fifth compounds, each of the fourth and fifthcompounds in said sub mixture being in substantially equal atomicamounts of from 0.01 to 10 atomic percent of said primary mixture withthe remainder of said sub mixture being said second compound; (g)melting said primary mixture and said sub-mixture to form a melt mass;(h) immersing a seed crystal into said melt mass; and (i) rotating andsimultaneously raising said seed crystal within said melt mass to form acrystalline boule.
 10. A method of growing a laser crystal as recited inclaim 9, wherein said melt mass is maintained at a temperature rangingfrom 750° C. to 850° C.
 11. A method of growing a laser crystal asrecited in claim 9, wherein said seed crystal is rotated at the ratefrom 16 rpm to 19 rpm.
 12. A method of growing a laser crystal asrecited in claim 9, wherein said seed crystal is raised at the rate ofless than 1 mm/hr.
 13. A method of growing a laser crystal of thecomposition M^(I) M^(II) M^(III) F₆ comprising the following steps:(a)providing a quantity of a first compound selected from the group LiF,NaF, KF, RbF, and CF; (b) providing a quantity of a second compoundselected from the group: BeF², MgF₂, CaF₂, SrF₂, CdF₂, and BaF₂ ; (c)providing a quantity of a third compound selected from the group: BF³,AlF₃, GaF₃, ScF₃, and InF₃ ; (d) providing a quantity of a fourthcompound of CeF₃ ; (e) providing a quantity of a fifth chargecompensating compound selected from the group: NaF, KF, MgF₂, BeF₂,CdF₂, and ZnF₂ ; (f) forming a mixture of substantially equal atomicunit amounts of said first, second and third compounds, and addingsufficient substantially equal atomic unit amounts of said fourth andfifth compounds to said mixture so that said combined second, third,fourth and fifth compounds have substantially double the atomic unitamounts of said first compound; (g) meting said mixture to form a meltmass; (h) immersing a seed crystal into said melt mass; and (i) rotatingand simultaneously raising said seed crystal within said melt mass toform a crystalline boule.
 14. A method of growing a laser crystal asrecited in claim 13, wherein the atomic percentage of each of saidfourth and fifth compounds is from 0.01 to 10 percent of said mixture.15. A method of growing a laser crystal as recited in claim 13, whereinsaid melt mass is maintained at a temperature ranging from 750° C. to850° C.
 16. A method of growing a laser crystal as recited in claim 13,wherein said seed crystal is rotated at the rate from 16 rpm to 19 rpm.17. A method of growing a laser crystal as recited in claim 13, whereinsaid seed crystal is raised at the rate of less than 1 mm/hr.